1. Field of the Invention
An present invention relates to an ion beam irradiation apparatus for implanting ions into a substrate by irradiating the substrate with an ion beam, more particularly relates to improvement for suppressing a charging (charge-up) of the substrate when it is irradiated with the ion beam.
2. Description of the Related Art
There is a proposal of suppressing a charge-up of a substrate when it is irradiated with ion beam. Plasma generated from a plasma generator is supplied to a region near the substrate. Electrons included in the plasma are used for neutralizing a positive charge generated by the ion beam irradiation. The proposed technique supplies electrons of lower energy to the substrate, when comparing with the technique utilizing secondary electrons emitted from an object when it is irradiated with electrons emitted from the filament. Accordingly, the proposed technique has an advantage of reducing the negative charge-up in the substrate.
A plasma production device of the radio frequency discharge type is the plasma production device using the radio frequency discharge for plasma generation. This type of the plasma production device is advantageously featured in that a) the lifetime is long because there is no filament, and b) it is operable at low gas pressure. When comparing with the plasma production device of the type which uses the filament for the discharge.
A sectional view showing the related art of an ion beam irradiation apparatus which is provided with a plasma production device of the radio frequency discharge type, is shown in FIG. 9.
An ion beam 2, which is shaped like a spot in cross section, is extracted from an ion source (not shown) in the ion beam irradiation apparatus. And if necessary, the ion beam is mass separated and accelerated before introducing in a vacuum chamber 8. In a vacuum chamber (process chamber) 8, the ion beam is irradiated onto a substrate (e.g., a semiconductor substrate) 4 held by a holder 6 so as to implant ions to the substrate 4 (ion implanting process), while being reciprocatively scanned in fixed directions X by the magnetic field (perpendicular to the surface of the drawing sheet of the figure, e.g., horizontal directions, which will be used hereinafter for the fixed directions).
The substrate 4 and the holder 6 are reciprocatively moved by a holder drive device 10 in a direction Y (e.g., vertical direction, which will be used hereinafter for the direction). The direction Y is substantially perpendicular to the directions X. This reciprocal scanning operation cooperates with the scanning of the ion beam 2 (hybrid-scan) to uniformly irradiate the entire surface of the substrate 4 with the ions.
A plasma production device 20 of the radio frequency discharge type is provided in the vicinity of the substrate 4 as viewed in the beam stream moving direction. The plasma production device 20 produces plasma 12 and supplies it to a region near to and in the vicinity of the substrate 4, whereby the charge-up of the surface of the substrate 4. The charging up is occurred by the irradiation of the ion beam 2.
The substrate 4 and the holder 6 are reciprocatively moved by a holder drive device 10 in directions Y (e.g., vertical directions, which will be used hereinafter for representing the corresponding directions). This reciprocal scanning operation cooperates with the scanning of the ion beam 2 (hybrid-scan) to uniformly irradiate the entire surface of the substrate 4 with the ions.
The plasma generator 20 of the radio frequency discharge type is provided at a position in the vicinity of the upstream side of the substrate 4 as viewed in the beam stream traveling direction. The plasma generator 20 generates a plasma 12 and supplies it to a region in the vicinity of the upstream side of the substrate 4, thereby suppressing the charging up of the surface of the substrate 4, which results from the irradiation of the ion beam 2. The plasma generator 20 is mounted on the outside of the vacuum vessel 8 located near the upstream side of the substrate 4, with the aid of an insulating member 30, for example.
The plasma generator 20 is provided with a cylindrical, plasma generating vessel 22. Gas (e.g., xenon gas) 14 is introduced into the plasma generating vessel, and an antenna 28 generates radio frequency discharge, which is generated by radio frequency electrical energy 18, into the gas-contained vessel; so that the introduced gas is ionized to generate plasma.
The generated plasma is emitted through a plasma emission hole 24. A magnetic coil 26 provided outside the plasma generating vessel 22 generates a magnetic field into the plasma generating vessel in the direction along an axis 23, which passes through the center of the plasma emission hole 24. The magnetic field is used for generating and maintaining the plasma 12.
Radio frequency electric power 18 is supplied from a radio frequency electric power source 16 to the antenna 28 by way of an impedance matching circuit 19. In the conventional technique, a waveform of the radio frequency electric power 18 output from the radio frequency electric power source 16 is a normal sinusoidal waveform, i.e., a continuous sinusoidal waveform having a fixed amplitude, and its frequency is 2.45 GHz or 3.56 MHz.
When the substrate 4 is irradiate with the ion beam 2, the surface of the substrate 4 is positively charged with the positive charge of the ion beam 2. In particular, in a case where the surface of the substrate 4 is covered with insulating material, it is easy to be charged. The plasma 12 is supplied to a region near the substrate 4 at the time when the ion beam is irradiated, as in the manner described above, electrons in the plasma 12 are attracted to the surface of the positively charged substrate 4, thereby neutralizing the positive charge of the surface of the substrate 4. If the positive charge is neutralized, the attraction of the electrons into the substrate 4 automatically stops as taught by the theory. In this way, the positively charge-up of the substrate surface by the ion beam irradiation is suppressed.
The neutralizing of the positive charges by the ion beam irradiation is carried out as described above. The surface of the substrate is positively or negatively charged for the following reasons.    1) In a state that the ion beam 2 is irradiating the substrate 4:
In this state, the surface of the substrate 4 is positively charged by the positive charge and emission of secondary electrons from the substrate 4. The emission of the secondary electrons is generated by the ion beam irradiation. At the same time, electrons of the plasma 12 generated by the plasma generator 20 are trapped in a beam plasma (actually, the ion beam 2 does not include only ions, but it is put in a plasma state since it traps electrons from its environment. This state is called the beam plasma). The ion beam plasma moves to the substrate 4 to neutralize the positive charge thereby and to relax the charging state in the substrate surface.
A level of the charge relaxation is determined by an electron density of the plasma 12 and electron energy thereof. The plasma is generated from the plasma generator 20.
Generally, the charge relaxation effect is large when the former is large, and the latter is low. The reason for this is that as the electron energy is lower, the beam plasma more easily traps electrons from the plasma 12 supplied from the plasma generator 20.    2) In a state that the substrate 4 is not irradiated with the ion beam 2:
Usually, the ion beam 2 is scanning, for scan, over a range exceeding the substrate 4 width (over-scan) . The substrate 4 is also moved in the directions Y, as described above. Accordingly, the time periods that the substrate 4 is not irradiated with the ion beam 2 are present during the emitting of the plasma 12 from the plasma generator 20. During the time period that the substrate is not irradiated with the ion beam, the substrate 4 is exposed to the plasma 12 emitted from the plasma generator 20. At this time, a charge-up voltage of the substrate surface is determined by a balance between the amount of electrons in the plasma 12 and the amount of ions in the plasma 12. Generally, the electron is lighter than the ion, and a mobility of electron is larger than that of the ion. Accordingly, the charge voltage of the substrate surface is negative in polarity.
For example, when ions are extremely small in amount in a region near the substrate 4, the charge voltage rises to a voltage corresponding to the maximum energy of electrons in the plasma 12.
As seen from the above description, to reduce the positive or negative charge-up voltage of the substrate 4, in particular, the negative charge-up voltage, it is necessary to reduce the electron energy in the plasma 12.
The plasma generator 20 of the radio frequency discharge type is capable of supplying electrons of low energy, when comparing with the technique using the primary electrons emitted from the filament and the secondary electrons emitted from an object when it is irradiated with the primary electrons. Recently, the technique of microfabrication of the semiconductor devices have made a great advance. In this situation, it is required that the charge-up voltage must be kept in low level during the ion implantation. The conventional technique is still unsatisfactory for satisfying such a requirement.
Even in the plasma generator 20 of the radio frequency discharge type, a radio frequency electric field is likely to accelerate electrons greatly when the plasma is generated, and hence, high energy electrons are readily generated. Use of the ECR (electron cyclotron resonance) discharge as one form of radio frequency discharge is useful in increasing plasma density, but electrons are considerably accelerated by the electron cyclotron resonance, and hence, higher energy electrons are generated in high possibility. As a result, the negative charge-up voltage of the substrate surface is likely to be high.
In this case, if the radio frequency electric power 18 supplied to the plasma generator 20 is reduced to be small, the electron energy in the plasma 12 is reduced. However, the density of the plasma 12 reduces, and the plasma 12 extinguishes. The approach of merely reducing the radio frequency electric power 18 is unsatisfactory for effective suppression of the charge of the substrate 4.